The present invention derives from the question of how to use commonly available arrays of diode lasers to pump simple rod shaped solid state lasers. In 2001-2002 the co-inventors developed and demonstrated pump cavities that would enable diode arrays to pump laser rods. The diode arrays are formed by stacking several diode laser bars, each typically 1 cm wide, with appropriate spacers. The emitting part of the array then covers a rectangular area which is seemingly incompatible with efficiently pumping a rod shaped solid state laser. In the experiments, changing pump arrays requires substantial time and re-alignment of the laser resonator particularly if donut arrays of diode pump lasers are used.
The co-inventors Ying Chen and Michael Bass later investigated pump cavities for diode laser array pumped laser rods including a new configuration of pump cavity for diode laser pumping solid state rod shaped lasers. Pump cavities are used to collect light from a pump source and deliver it to a gain medium where it can be absorbed and excite the medium to lase. Diode lasers are excellent pump sources but the light diverges and is hard to get into the gain medium. Rod shaped solid state laser media are very convenient to manufacture and to seal for cooling. Arrays of diode lasers are commercially available. The inventors developed a pump cavity with a house shaped cross section called a roof top cavity that is convenient to produce, compatible with commercially available diode laser arrays.
The work performed by the co-inventors included the concept of the “D” and “roof-top” laser pump cavities. It is worth noting that in his book, “Solid State Laser Engineering”, Walter Koechner describes face pumping slab lasers with flashlamps in pump cavities that reverse the co-inventors concepts. The lamp is placed on the axis of the “D” or the “roof-top” (Koechner calls this a “V” pump cavity) and the slab is placed in the base plane. In the configuration developed by the co-inventors, the gain medium was placed where Koechner has the flashlamp and an approximately planar array of pump light was placed in the base plane where Koechner has the gain medium enabling pumping rod shaped lasers with diode arrays. Tests were conducted with this configuration.
FIGS. 1a and 1b show a “D” 100 and a “V” 150 cavity, respectively, including cross sections and dimensions. The diode bars 120 and 170 were oriented in the base plane parallel to the length of the gain medium. For the D-type cavity there are two rod positions 110 and 115 considered during the testing in the “D” cavity 100 as shown in FIG. 1. One rod 115 is along the axis of the cylindrical section and places the rod 115 axis 13 mm from the plane of the diode array 120. This example is referred to as the case of a centered rod. The other rod 110 position is called off centered and locates the rod 115 center 16 mm from the plane of the diode array 120. For the V-type cavity 150 there is one position shown for the rod 160 which is placed 14 mm from the plane of the diode array 170.
Four kinds of surfaces or materials were used in both “D” and “V” cavities for the experiments. These are Spectralon, and metal surfaces we call S1, S2, and S3. The S1surface is the one with fine size EDM finishing, S2 has coarse size EDM finishing and S3 is as finished in the milling machining. All three metal surfaces were gold plated. Using the Cary 500 reflectance measurement capability it was found that the S1 and S2 surfaces reflect about 96% while the Spectralon reflects about 98%. The S3 surface was not measured since it presented both diffusive and specular reflection.
Using an experimental setup, the pattern of fluorescence, which is indicative of the distribution of the absorbed pump power, was observed. The fluorescence pattern for each “D” or “V” pump cavity was measured. Both the “D” and “V” pump cavities led to respectable lasing but the efficiency achieved was not what had been expected. To achieve higher efficiency the co-inventors determined that they must eliminate any gaps between the diode array and the pump cavity walls that were present in these experiments and that further research was required to understand the properties of the pump cavity walls as they reflect, scatter or absorb pump light. It became clear that no matter where you placed the rod or how you finished the inner walls of either type of cavity, the rod's absorption of pump light would not be both efficient and uniform.
Thus, the need exists for solutions to the above problems with the prior art. The co-inventors have developed a pump cavity with a house shaped cross section called a roof top cavity that is convenient to produce, compatible with commercially available diode laser arrays, and resulted in efficient pump light absorption with good uniformity.